Conventional transport aircraft typically utilize pneumatic, hydraulic, and electric power from main engines to support various aircraft systems during flight. In addition, conventional transport aircraft typically utilize pneumatic and electric power from on-board auxiliary power units (APUs) to support aircraft systems during ground operations. Aircraft air conditioning systems are typically the largest secondary power users on commercial transport aircraft. On conventional transport aircraft, these systems use high temperature/high pressure air extracted from the engine compressor stages (“bleed air”). The air passes through air conditioning packs before passing into the fuselage to meet temperature, ventilation, and pressurization needs. The conditioned air is then discharged from the fuselage through outflow valves or through normal cabin leakage. During ground operations, the APU can provide bleed air either from a separate shaft-driven load compressor or from a power section compressor. Similar to the bleed air from the main engines, the high temperature and high pressure air from the APU passes through air conditioning packs before passing into the fuselage.
FIG. 1 schematically illustrates a conventional pneumatic-based secondary power system architecture 100 configured in accordance with the prior art. The system architecture 100 can include jet engines 110 (shown as a first engine 110a and a second engine 110b) for providing propulsive thrust to the aircraft (not shown). In addition to thrust, the engines 110 can also provide high temperature/high pressure air to a bleed manifold 120 via bleed ports 112 (identified individually as a first bleed port 112a and a second bleed port 112b). The bleed ports 112 receive air from the compressor stages of the engines 110, and pass the air through heat exchangers 114 (such as precoolers) that cool the air before it passes to the bleed manifold 120.
The high pressure air from the bleed manifold 120 supports the majority of secondary power needs of the aircraft. For example, a portion of this air flows to air conditioning packs 140 (shown as a first air conditioning pack 140a and a second air conditioning pack 140b) that supply conditioned air to a passenger cabin 102 in a fuselage 104. The air conditioning packs 140 include a series of heat exchangers, modulating valves, and air cycle machines that condition the air to meet the temperature, ventilation, and pressurization needs of the passenger cabin 102. Another portion of air from the bleed manifold 120 flows to turbines 160 that drive high capacity hydraulic pumps 168. The hydraulic pumps 168 provide hydraulic power to the landing gear and other hydraulic systems of the aircraft. Yet other portions of this high pressure air are directed to an engine cowl ice protection system 152 and a wing ice protection system 150.
The wing ice protection system 150 includes a valve (not shown) that controls the flow of bleed air to the wing leading edge, and a “piccolo” duct (also not shown) that distributes the hot air evenly along the protected area of the wing leading edge. If ice protection of leading edge slats is required, a telescoping duct can be used to supply hot bleed air to the slats in the extended position. The ice protection bleed air is exhausted through holes in the lower surface of the wing or slat.
In addition to the engines 110, the system architecture 100 can also include an APU 130 as an alternate power source. The APU 130 is typically started by a DC starter motor 134 using a battery 136. The APU 130 drives a compressor 138 that provides high pressure air to the bleed manifold 120 for engine starting and other ground operations. For engine starting, the high pressure air flows from the bleed manifold 120 to start-turbines 154 operably coupled to each of the engines 110. As an alternative to the APU 130, bleed air from a running one of the engines 110 can be used to re-start the other engine 110. As a further alternative, an external air cart (not shown in FIG. 1) can provide high pressure air for engine starting on the ground.
The system architecture 100 can further include engine-driven generators 116 operably coupled to the engines 110, and an APU-driven generator 132 operably coupled to the APU 130. In flight, the engine-driven generators 116 can support conventional electrical system loads such as a fuel pump 108, motor-driven hydraulic pumps 178, and various fans, galley systems, in-flight entertainment systems, lighting systems, avionics systems, and the like. The APU-driven generator 132 can support these functions during ground operations and during flight as required. The engine-driven generator 116 and the APU-driven generator 132 are typically rated at 90–120 kVA and produce a voltage of 115 Vac. They can provide power to transformer-rectifier units that convert 115 Vac to 28 Vdc for many of the abovementioned electrical loads. The power is distributed through an electrical system based largely on thermal circuit-breakers and relays.
The system architecture 100 can additionally include engine-driven hydraulic pumps 118 operably coupled to the engines 110. The hydraulic pumps 118 provide hydraulic power to control surface actuators and other aircraft systems in flight. Electric-motor driven pumps 178 can provide back-up hydraulic power for maintenance activities on the ground.
FIG. 2 is a schematic top view of a prior art aircraft 202 that includes the secondary power system architecture 100 of FIG. 1. The aircraft 202 includes a forward electronic equipment bay 210 that distributes electrical power to a plurality of electrical loads 220 associated with the system architecture 100 described above. In flight, the electronic equipment bay 210 can receive electrical power from the engine generators 116, as well as the APU 130. On the ground, the electronic equipment bay 210 can receive electrical power from the APU 130, or from an external power source 212 via a receptacle 213.
One shortcoming of the secondary power system architecture 100 described above is that it is sized for a worst case operating condition (typically, cruise speed, high aircraft load, hot day, and one engine bleed air system failed) to ensure sufficient air flow is available to meet system demands at all times. As a result, under typical operating conditions, the engines 110 provide bleed air at a significantly higher pressure and temperature than the air conditioning packs 140 and the other aircraft systems demand. To compensate, the precoolers 114 and the air conditioning packs 140 regulate the pressure and temperature to lower values as required to meet the demands for fuselage pressurization, ventilation, and temperature control. Consequently, a significant amount of energy is wasted by precoolers and modulating valves during this regulation. Even under optimum conditions, a significant amount of energy extracted from the engines 110 is wasted in the form of heat and pressure drops that occur in the ducting, valves and other components associated with the bleed manifold 120 and the air conditioning packs 140.